1. Field of the Invention
The present invention relates to a rotary drive device used as a motor having high rotational speed with high accuracy, such as a motor for driving a rotary polygon mirror which is used in a recording system such as a laser beam printer (LBP), a motor for driving a video tape recorder or video disc player, or the like.
2. Description of the Prior Art
It is known that an image recording system obtaining prints based on digital signals, such as LBP, digital copier or the like, generally utilizes a laser scan type optical system as shown in FIG. 1.
Such an optical system comprises a laser unit 1 actuated and stopped in accordance with digital signals, a rotary polygon mirror 2 rotatable about a shaft 6 for deflecting and scanning the laser light along the axial direction of the shaft 5 of a drum 4 having a photosensitive layer which is formed therein at the outer periphery, and a lens system 3. A latent image formed on the photosensitive layer by the laser scanning light is succeedingly subjected to development, transfer and fixation to form a visible image, in accordance with an electrophotographic process such as Carlson process, PIP process or others.
Among the components of the optical system, the rotary polygon mirror 2 is the most important component. It cannot be attained to stabilize and improve the image in quality unless the polygon mirror 2 is rotated with higher accuracy. A motor for driving the rotary polygon mirror is therefore placed under severe conditions, particularly with respect to the following requirements which are also true of motors for VTR or others:
1. Reduced irregularity in rotation; PA1 2. Reduced deflection of shaft; PA1 3. No scattering of grease or oil from bearings; PA1 4. Revolution higher than 4000 r.p.m. in general; and PA1 5. Rotation for a long period of time without reducing the accuracy and without any noise.
To fulfill the above requirements, the prior art employed a motor for directly driving the polygon 2 as shown in FIG. 2. This motor comprises a shaft 12 on which high-precision bearings 7 and 11, a rotor magnet 8 and balance rings 15 and 16 are press-fitted or bonded. The upper bearing 7 is bonded to a holder 30 which is in turn fixed to a closure 17. The lower bearing 11 is mounted in a motor casing 18 movable in the vertical direction. The lower bearing 11 is upwardly urged by a spring 13 located between the outer race of the bearing 11 and a spring seat 35 which is fixed to the motor casing 18, so that the lower bearing 11 can firmly be positioned without any play and/or deflection. There is a stator 9 located around the rotor magnet 8 and spaced outwardly therefrom. Below the stator 9 there is located a Hall element 10. The rotor magnet 8, stator 9 and Hall element 10 define the so-called D.C. Hall motor. The balance ring 15 includes a black-and-white pattern printed thereon at the outer periphery which pattern is detected by means of a reflection type photo-coupler 14 to obtain signals for controlling the revolution of the rotor magnet 8.
The polygon 2 is sealed by a cap 19 which is mounted on the closure 17. The cap 19 includes a plurality of openings 20 formed therein at the peripheral wall thereof and closed by glass sheets. Laser beam is adapted to enter and exit the cap 19 through the openings 20. There is a magnetic seal 28, a mechanical seal such as labyrinth or the like which prevents the mirror faces of the polygon from subjecting to contamination due to any scattering of grease from the bearing.
There is also known another type of motor which is an outer rotor type D.C. motor directly connected to a spindle having bearings at the opposite ends (bearing unit). In any event, the rotary polygon mirror, bearings and the rotor for driving the mirror (rotor magnet 8 in FIG. 1) are mounted on a single shaft at positions spaced away from each other in the longitudinal direction. Accordingly, the system cannot be made compact. Since the upper and lower bearings are centered by the use of many separately constructed components (the holder 30, closure 17 and motor casing 18 in FIG. 1), the deflection in the shaft cannot completely be eliminated. If the rotational speed of the motor is increased, it is desirable that the motor is lubricated by oil in the form of droplets rather than the enclosed grease in view of the life of bearings. However, this cannot be performed in the above motor construction. That is, the electronic components, for example, such as Hall element 10 and the photo-coupler 14 will be possibly contaminated with grease in such a construction.